Dye Diffusion Thermal Transfer

Within the last five to seven years, printhead development has made dye diffusion thermal transfer digital printing technology a viable choice for card substrates. Consequently dye diffusion thermal transfer is experiencing rapid growth in the card sector because of its ability to render high-quality, on-demand color photographic images. The image formation technique is similar to thermal transfer color printing in that the technology superimposes the three subtractive primary colors —yellow, cyan, and magenta— and possibly black (ribbon comes in either three-, four-, or five-color configurations) in a single image field to produce almost any color. However, in the dye diffusion thermal transfer printing process, the three color ribbon is dye-impregnated, and the receiving substrate is coated with a special layer to receive and fix the dye ink.

The optional, black, fourth panel is carbon based thermal transfer to produce high definition fine print and infrared (IR) scannable barcodes. Five-color ribbons incorporating a fifth (clear) hot stamp thermal transfer varnish are used to help protect the somewhat fragile images. For frequently used cards, a thin protective film is laminated over the image. The laminating film is supplied as a cartridge separate from the color ribbon, and frequently carries optically variable inks, holograms, and other fraud deterring devices.

A printhead with heating elements heats points on the ribbon, causing the dye to sublimate, i.e., to pass from a solid to a gaseous state without an intervening liquid phase. The treated receiver plastic card substrate then absorbs the vaporized dye. The amount of heat produced by the head can be varied to diffuse different amounts of dye into the substrate, thus creating pixels of different intensities to enable continuous-tone output.

Card blanks, ready for dye diffusion thermal transfer images, may be plain, unprinted, or may have nonvariable images and information preprinted and laminated within (similar to credit cards). Most blanks are low cost PVC. Cards for longer life are available and are made from either Polyphenyl delta-Butylene (PdB) or vinyl/polyester composites.

Currently dye diffusion thermal transfer is considered a high-end card ID technology. Given the widespread use of cheaper film-based ID systems, it will not supplant the competition any time soon. However, dye diffusion thermal transfer has one critical advantage over film-based systems in that digital images and signatures can be electronically stored and/or transmitted. This characteristic makes it the technology of choice for motor vehicle IDs and any card application that involves archiving and communication of personal identification data.

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